This invention relates generally to wind turbines, and more particularly to a system and method for efficiently controlling wind turbines during grid loss and a change in wind conditions, especially under storm winds, to maintain the rotor speed at a substantially constant value to keep the uninterruptable power supply (UPS) active.
Recently, wind turbines have received increased attention as an environmentally safe and relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted within a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 80 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox. The gearbox may be used to step up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid. Some turbines utilize generators that are directly coupled to the rotor without using a gearbox.
The design of the blades and the tower of a wind turbine are often dimensioned by the extreme loads that occur during storm winds combined with grid loss. Even though the turbine blades are fixed at an angle close to ninety (90) degrees, they are not really in a feathered position because the lack of grid power prevents the wind turbine from yawing towards the wind direction. Extreme loads in the blades and tower are produced by the force of strong storm winds that hit a large surface area of the blade (lateral yaw direction). The storm loads may be alleviated by providing a source of secondary power, such as a diesel generator, in order to yaw the turbine towards the incoming wind. For example, one such method keeps the plane of rotation of the rotor substantially perpendicular to the direction of wind. The blade angle of the rotor blades are adjusted to a minimum operating angle close to ninety (90) degrees for spinning the rotor and the generator to produce the necessary power to turn the rotor and to keep the rotor toward the incoming wind during storm loads. However, it is desirable to alleviate the need for a separate backup generator because of the extra cost and complexity associate therewith.
In addition, the yawing of the wind turbine to keep the rotor perpendicular to the wind direction may cause undesirable cable twisting as the wind direction changes. Therefore, there is a limit on how much the wind turbine can yaw, which is usually at most about three full rotations in either direction. In addition, the yawing the turbine can only be done very slowly (about 0.5 degrees/s) due to load constraints on the turbine structure. Hence, if the wind direction changes rapidly, one will not be able to keep the rotor perpendicular to the wind direction and the strategy used in conventional wind turbines will not have the desired outcome. Therefore, it is desirable to alleviate the storm loads without the need to yaw the turbine towards the incoming wind.